This commit replaces the basic placeholder with a full-featured demo page showcasing Tailwind CSS v4 capabilities.
- Implements a modern, responsive landing page layout in `TailwindcssDemo.vue`.
- Defines a custom primary color palette and a float animation using the new `@theme` at-rule in `main.css`.
- Adds a custom `.text-gradient` utility class.
- Updates the router to nest the demo page, improving scalability for future routes.
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